When you say "lines" it is difficult to say as there are many types of lines.

The most common lines seen are longitudinal (running along the length of the nail). More specifically, longitudinal melanonychia means dark lines along the nails. This can be variant of normal, particularly in persons with dark skin. However, any new line of pigmentation should be evaluated by a dermatologist.

In brief:
Lots of reasons

When you say "lines" it is difficult to say as there are many types of lines.

The most common lines seen are longitudinal (running along the length of the nail). More specifically, longitudinal melanonychia means dark lines along the nails. This can be variant of normal, particularly in persons with dark skin. However, any new line of pigmentation should be evaluated by a dermatologist.
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